In recent years, lot types of projection device such as projectors are widely used in families, schools and business occasions in order to amplify and display an image signal provided by an image source on a screen. For the purpose of reducing the power consumption and the product size, a solid-state light-emitting element (e.g. a LED or a laser element) is employed in the illumination system of current projector to replace the conventional high intensity discharge (HID) lamp.
In general, the illumination system of the projector should emit three primary color lights, i.e. red light (R), green light (G) and blue light (B). As for the luminous efficiency of the solid-state light-emitting element, the blue solid-state light-emitting element has higher luminous efficiency than the red solid-state light-emitting element or the green solid-state light-emitting element does. Since the blue solid-state light-emitting element has better luminous efficiency, the red light and the green light are produced by using a blue solid-state light-emitting element and a wavelength-transforming device (e.g. a phosphor wheel or a phosphor plate) to excite blue light as red light or green light. That is, without the red solid-state light-emitting element and the green solid-state light-emitting element, the uses of the blue solid-state light-emitting element and the device containing phosphor coating can directly emit the red light or the green light. Consequently, the luminous efficiency of the whole illumination system is enhanced.
For example, one of the illumination systems of the conventional projection device utilizes plural blue solid-state light-emitting elements and plural phosphor wheels having a single region, and the other one of the illumination systems of the conventional projection device utilizes a single solid-state light-emitting element and a single phosphor wheel having plural regions. Please refer to FIG. 1A and FIG. 1B. FIG. 1A schematically illustrates the structure of a conventional phosphor wheel having a single region. FIG. 1B schematically illustrates the structure of a conventional phosphor wheel having plural regions. As shown in FIG. 1A, a conventional phosphor wheel 10 has a single region 101. A type of phosphor agent is coated on the single region 101 (i.e. the whole circle of phosphor wheel 10), wherein this phosphor agent is composed of plural kinds of phosphor powders and called mixture-powders phosphor agent. As shown in FIG. 1B, another conventional phosphor wheel 11 includes plural regions, and the number of the regions is 3 or 4 in general. An example of the conventional phosphor wheel 11 includes a first region 111, a second region 112, a third region 113 and a fourth region 114. A red type of phosphor agent, a green type of phosphor agent, a blue type of phosphor agent and a yellow type of phosphor agent are respectively coated on the first region 111, the second region 112, the third region 113 and the fourth region 114, wherein each type of phosphor agents is composed of plural kinds of phosphor powders and called mixture-powders phosphor agent.
To increase the light color diversity of the phosphor wheel, a phosphor agent is usually composed of two or more phosphor powders for adjusting the characteristics of the output light. For example, a red phosphor powder is usually added for the purpose of outputting warmer color lights, or a high-luminance phosphor powder having a main output wavelength of 555 nanometers is usually added for the purpose of increasing the luminance value of the output light. Nevertheless, different kinds of the phosphor powders may be fabricated by different companies, so it is not easy to know the exact composition of each phosphor powder. Under this circumstance, it is relatively difficult to mix the phosphor powders for obtaining the target phosphor agent. Also, a cascade effect occurs because of the different materials of the different phosphor powders. That is, a high energy level of one kind of phosphor powder is absorbed by the phosphor powder having a low energy level (i.e. long-wavelength phosphor material), so that the total output efficiency is decreased.
A common cascade effect occurs when mixing a yellow phosphor powder and a red phosphor powder. Because the part of output yellow light, which has a wavelength range from 510 nanometers to 580 nanometers, is absorbed by the red phosphor powder, the area of the spectrum of total output light (i.e. the output energy) is lower than the theoretical value of mixing the yellow phosphor powder and the red phosphor powder by 14%. As a result, not only the theoretical value cannot be achieved by the conventional powder-mixing method of prior art, but also the color saturation and hue cannot be matched with the industry standard value, such as the standard value of Rec. 709 (or called ITU-R Recommendation BT. 709) gamut of High Definition Television (HDTV), due to the cascade effect.
There is a need of providing a phosphor device and an illumination system using the same to obviate the drawbacks encountered from the prior art.